For the past several years a number of new developments have occurred in manufacturing processes for composite panel structures by the use of thermal set adhesives. More specifically, in the automotive field, the use of thermal set adhesives is being investigated for the manufacture of body component assemblies such as doors, trunks and deck lids in place of conventional spot weld and resistance welding techniques. Generally speaking, one of the current techniques being investigated is to place the adhesive on one of the panels and accurately position the other panel (or panels with adhesive also therebetween) by means of especially made jigs and fixtures. Portions of the panels overlying the adhesive are then heated by induction heating to cause heat to be transferred to the adhesive or alternatively, the adhesive, itself, may be heated. In either event, the adhesive is thermally set so that the assembled components are held together in a reasonably firm manner for the remainder of the finishing operations. Depending on the type of adhesive used, a firm bond results either as a function of time and/or when the body is exposed to an elevated curing temperature in the paint baking oven. One such arrangement disclosing this technique in greater detail may be found in U.S. Pat. No. 4,602,139 to Hutton et al which is incorporated herein by reference in its entirety.
A number of difficulties have been experienced with respect to setting the thermal adhesive. These difficulties principally arise from either applying too much heat, or too little heat or too much of an uneven heat to the adhesive joint which either results in failure to set the adhesive or in overheating of the adhesive which may adversely affect its retention properties or even, in some instances, a distortion in the sheet metal itself as a result of the normal thermal expansion and contraction of the sheet metal. Several approaches to solve such problems have centered about various jig and fixture designs which not only hold the panels in their preferred position but also maintain the inductor coil air gap at its proper position. Other proposed solutions have attempted to control the power to the induction coil and/or frequency thereof. Still other proposed solutions have been to investigate and formulate different adhesives which could better withstand the variations in heat encountered in the assembly process. For example, U.S. Pat. Nos. 4,650,947 and 4,654,495 to Hutton et al also incorporated by reference herein disclose a two-stage adhesive with induction heating used to trigger the first "tack" curing stage while the permanent bond is established in the second stage when a blowing agent is activated to produce a foam resin. All such solutions have met with limited degrees of success. None of the solutions is universally acceptable.
The use of induction heating for bonding thermal set adhesives is significantly limited when bonding a metal panel to a plastic panel or a plastic panel to a plastic panel. Such applications exist not only in the automotive field but in countless other fields as well. Since there is little if any molecular action within plastics induced by an alternating field which can generate heat, attempts to adhesively bond plastic to plastic or plastic to metal by induction heating have been to mix small metal flakes in the adhesive so that the metal flakes can be inductively heated to set the adhesive. This approach has achieved only limited success because of the tendency of the metal flakes to become non-uniformly dispersed within the adhesive resulting in uneven induction heating causing certain adhesive portions to become overheated and other portions to be insufficiently heated. From a conceptual analysis, the suspension problem is difficult if not impossible to correct. The problem is significantly aggravated when a thermal bond is to be effected between a metal and a plastic component. In such application, the induced flux tends to heat the metal component more than the metal flakes to promote overheating that portion of the adhesive in contact with the metal component. This in turn requires the positioning of the coil and the frequencies at which the induction coil operates to be closely controlled to regulate the depth of heating effected by the coil.
Apart from any considerations of induction heating, it can be appreciated that if the space between the panels is not flush or if the spacing filled by the adhesive is significantly varied, the curing or set time of the adhesive can be adversely affected, simply because a greater mass of adhesive is involved. This problem occurs in especially configured panels where corners are involved or where bends might produce spaces which the adhesive must fill so that the processing time is longer than that which might otherwise be achieved.